Embodiments of the present invention relate to a low noise, current mode capacitive sensor having an integrated bandpass filter.
Touch screen input panels are widely used in cell phones, computer tablets, laptop and desktop computers, video games, and other electronic devices. These panels may be resistive type, light sensing type, or capacitive sensing type. Referring to FIG. 1, there is a capacitive sensing touch screen input system of the prior art as disclosed by Tang et al. (U.S. Pat. No. 8,477,120). The touch screen system includes an array of touch screen cells formed over a liquid crystal display (LCD). The touch screen cells are typically formed where rows of drive electrodes are insulatively disposed proximate columns of sense electrodes. Each drive electrode includes a parasitic capacitance CDX 102. Here, X is a row number from 0 to N−1 where the array includes N rows. Likewise, each sense electrode includes a parasitic capacitance CSY 106. Here, Y is a column number from 0 to M−1 where the array includes M columns. A mutual capacitance is formed at each intersection of a row and column. For example, mutual capacitance CMXY 104 is formed at the intersection of row X and column Y. A drive signal generator 100 generates drive voltage VD on row X, which is applied through mutual capacitance CMXY 104 to sense amplifier 108. When the touch screen cell is touched by a finger or high permittivity stylus, mutual capacitance CMXY 104 increases and produces a high level input signal Vin at sense amplifier 108. Sense amplifier 108 subsequently amplifies this input signal and applies it to processor 110 where it is processed by application software.
There are several problems, however, with this touch screen system. First, row and column selection in the touch screen array produces significant array noise that is coupled to sense amplifier 108. Second, row and column electrodes of the touch screen array act as small antennas and transmit electromagnetic interference (EMI) from various sources such as microwave ovens and electrical appliances to sense amplifier 108. Finally, sense amplifier 108 produces it's own noise. Thus, it is desirable to include a bandpass filter in sense amplifier 108 to attenuate EMI. Turning to FIG. 1B, there is a sense amplifier with integrated bandpass filter of the prior art. The circuit includes a transimpedance amplifier (TIA) coupled in series with a Sallen-Key amplifier configured as a bandpass filter. Laplace transforms of transfer functions of the TIA and the Sallen-Key bandpass amplifier are given by equations [1] and [2], respectively, where K is 1+Rf2/Rf3. The transfer function for the circuit of FIG. 1B, therefore, is a product of equations [1] and [2].
                                          H            ⁡                          (              s              )                                TIA                =                              sC            ⁢                                                  ⁢            0            ⁢            Rf            ⁢                                                  ⁢            1                                              sCf              ⁢                                                          ⁢              1              ⁢              Rf              ⁢                                                          ⁢              1                        +            1                                              [        1        ]                                                      H            ⁡                          (              s              )                                            S            -            K                          =                              Ks                          R              ⁢                                                          ⁢              1              ⁢              C              ⁢                                                          ⁢              1                                                                                                                s                    2                                    +                                      s                    ⁢                                          (                                                                        1                                                      R                            ⁢                                                                                                                  ⁢                            1                            ⁢                            C                            ⁢                                                                                                                  ⁢                            1                                                                          +                                                  1                                                      R                            ⁢                                                                                                                  ⁢                            2                            ⁢                            C                            ⁢                                                                                                                  ⁢                            1                                                                          +                                                  1                                                      R                            ⁢                                                                                                                  ⁢                            2                            ⁢                            C                            ⁢                                                                                                                  ⁢                            2                                                                          -                                                                              Rf                            ⁢                                                                                                                  ⁢                            2                                                                                Rf                            ⁢                                                                                                                  ⁢                            3                            ⁢                            RfC                            ⁢                                                                                                                  ⁢                            1                                                                                              )                                                        +                                                                                                                                                R                      ⁢                                                                                          ⁢                      1                                        +                    Rf                                                        R                    ⁢                                                                                  ⁢                    1                    ⁢                    RfR                    ⁢                                                                                  ⁢                    2                    ⁢                    C                    ⁢                                                                                  ⁢                    1                    ⁢                    C                    ⁢                                                                                  ⁢                    2                                                                                                          [        2        ]            
The center frequency and bandwidth of the circuit of FIG. 1B are given below in equations [3] and [4], respectively. The gain and Q factor are given by equations [5] and [6], respectively.
                              ω          0                =                                                            R                ⁢                                                                  ⁢                1                            +              Rf                                      R              ⁢                                                          ⁢              1              ⁢              RfR              ⁢                                                          ⁢              2              ⁢              R              ⁢                                                          ⁢              1              ⁢              C              ⁢                                                          ⁢              2                                                          [        3        ]                                BW        =                              1                          R              ⁢                                                          ⁢              1              ⁢              C              ⁢                                                          ⁢              1                                +                      1                          R              ⁢                                                          ⁢              2              ⁢              C              ⁢                                                          ⁢              1                                +                                    1              Rf                        ⁢                          (                              1                -                K                            )                                +                      1                          R              ⁢                                                          ⁢              2              ⁢              C              ⁢                                                          ⁢              2                                                          [        4        ]                                GAIN        =                              KC            ⁢                                                  ⁢            0                                R            ⁢                                                  ⁢            1            ⁢            C            ⁢                                                  ⁢            1            ⁢            Cf            ⁢                                                  ⁢            1            ⁢            BW                                              [        5        ]                                Q        =                                                            (                                                      R                    ⁢                                                                                  ⁢                    1                                    +                                      R                    ⁢                                                                                  ⁢                    3                                                  )                            ⁢              R              ⁢                                                          ⁢              1              ⁢              R              ⁢                                                          ⁢              3              ⁢              C              ⁢                                                          ⁢              1              ⁢              C              ⁢                                                          ⁢              2                                                          R              ⁢                                                          ⁢              1              ⁢              R              ⁢                                                          ⁢              3              ⁢                              (                                                      C                    ⁢                                                                                  ⁢                    1                                    +                                      C                    ⁢                                                                                  ⁢                    2                                                  )                                      +                          R              ⁢                                                          ⁢              2              ⁢              C              ⁢                                                          ⁢              2              ⁢                              (                                                      R                    ⁢                                                                                  ⁢                    3                                    -                                                                                    Rf                        ⁢                                                                                                  ⁢                        2                                                                    Rf                        ⁢                                                                                                  ⁢                        3                                                              ⁢                    R                    ⁢                                                                                  ⁢                    1                                                  )                                                                        [        6        ]            
There are several significant limitations of the circuit of FIG. 1B. Both bandwidth (BW) and center frequency (ω0) depend on passive components R1, R2, Rf, C1, and C2. By way of comparison, GAIN depends on K, C0 and inversely on R1, C1, Cf, and BW. Thus, minor process variations may significantly alter circuit performance. Although amplifier GAIN increases with K, it can not be very high or the circuit becomes unstable. Moreover, when K is greater than 1, the circuit does not operate with unity feedback, and total harmonic distortion (THD) increases.
While preceding approaches have provided improvements in touch screen sensing and noise reduction, the present inventors recognize that still further improvements are possible. Accordingly, the preferred embodiments described below are directed toward improving upon the prior art.